Abstract The Choquequirao Formation is a >3 km-thick amphibolite-grade succession that outcrops in the Central Andes of southern Peru. To constrain its age and tectonostratigraphic setting, detrital zircon and metamorphic zircon, titanite, and rutile U–Pb isotopic analyses were conducted. Mantle-derived c. 640 Ma detrital zircons constrain the maximum age of the lower part of the succession and 550–490 Ma metamorphic zircon domains constrain its minimum age. The absence of early Paleozoic detrital zircons suggests that deposition predated early Paleozoic orogenesis in southwestern Gondwana. The close similarity of detrital zircon age spectra to those from sediments deposited on the Arequipa basement suggests that the Choquequirao Formation was deposited on the Arequipa Terrane. Metamorphic titanite dates are highly overdispersed, yet they overlap with c. 460 Ma peak metamorphism recorded by metamorphic zircon. Pb-loss pathways displayed by metamorphic titanite have a lower intercept that overlaps with c. 325 Ma metamorphic rutile, which corresponds to Hercynian orogenesis. A poorly constrained upper intercept of c. 510 Ma may correspond to Pampean and/or early Famatinian orogenesis. We interpret the Cryogenian–Ediacaran Choquequirao Formation as having been deposited during the opening of the Palaeo-Iapetus (Puncoviscana–Clymene) Ocean between eastern Arequipa and southern Kalahari prior to the subsequent collision with southwestern Amazonia during the Pampean Orogeny.

ABSTRACT This field guide presents a one-day trip across the northern Virginia Blue Ridge geologic province and highlights published geologic mapping of Mesoproterozoic rocks that constitute the core of the Blue Ridge anticlinorium and Neoproterozoic cover-sequence rocks on the fold limbs. The guide presents zircon SHRIMP (sensitive high-resolution ion microprobe) U-Pb crystallization ages of granitoid rocks and discusses the tectonic and petrologic evolution of basement rocks during the Mesoproterozoic. U-Pb data show more of a continuum for Blue Ridge Mesoproterozoic magmatic events, from ca. 1.18–1.05 Ga, than previous U-Pb TIMS (thermal ionization mass spectrometry)-based models that had three distinct episodes of plutonic intrusion. All of the younger dated rocks are found west of the N-S–elongate batholith of the Neoproterozoic Robertson River Igneous Suite, suggesting that the batholith occupies a fundamental Mesoproterozoic crustal boundary that was likely a fault. Narrow belts of paragneiss may represent remnants of pre-intrusive country rock, but some were deposited close to 1 Ga according to detrital zircon U-Pb ages. For late Neoproterozoic geology, the guide focuses on lithologies and structures associated with early rifting of the Rodinia supercontinent, including small rift basins preserved on the eastern limb of the anticlinorium. These basins have locally thickened packages of clastic metasedimentary rocks that strike into and truncate abruptly against Mesoproterozoic basement along apparent steep normal faults. Both basement and cover were intruded by NE-SE–striking and steeply dipping, few-m-wide diabase dikes that were feeders to late Neoproterozoic Catoctin Formation metabasalt that overlies the rift sediments. The relatively weak dikes facilitated the deformation that led to the formation of the Blue Ridge anticlinorium during the middle to late Paleozoic as the vertical dikes were transposed and rotated during formation of the penetrative cleavage. This field trip generally follows the GSA guide published in GSA Field Guide 57 (available at https://pubs.geoscienceworld.org/gsa ): Burton, W.C., and Schindler, J.S., 2020, Proterozoic and Paleozoic evolution of the Blue Ridge geologic province in northern Virginia, USA, in Swezey, C.S., and Carter, M.W., eds., Geology Field Trips in and around the U.S. Capital: Geological Society of America Field Guide 57, p. 1–19, https://doi.org/10.1130/2020.0057(01) .

ABSTRACT The Mesoproterozoic southeastern margin of Laurentia, which consisted primarily of the ca. 1.5–1.35 Ga Granite-Rhyolite Province, was extensively reworked during ca. 1.3–0.9 Ga phases of the Grenville orogenic cycle. Questions remain for much of southeastern Laurentia regarding the transition from the Granite-Rhyolite Province to Grenville orogenic cycle, and for potential collisional interaction with Amazonia, due to Paleozoic sedimentary cover or tectonic reworking. Basement rocks sampled by drill core in the east-central United States include 1.5–1.35 Ga magmatic rocks, some overprinted by late Geon 10 (Ottawan) orogenesis, which are the most outboard evidence of Granite-Rhyolite Province crust. Newly recognized 1.35–1.30 Ga (pre-Elzevirian) granitic orthogneisses within the Mars Hill terrane of southeastern Laurentia (1) expand the along-strike distribution of the earliest crustal age components of the Grenville orogenic cycle in Appalachian basement inliers; (2) contain Geon 19–16 inherited zircons; and (3) were metamorphosed during late Ottawan to Rigolet tectonism. Paragneisses enveloping the Geon 13 orthogneisses are dominated by Geon 19–16 and Geon 13–12 detrital zircons overgrown by Geon 10–9 metamorphic zircon. The zircon age systematics require the paragneiss protoliths to be younger than orthogneiss protoliths and be partly sourced from the latter. Orthogneisses and paragneisses have Pb isotope compositions that overlap those of south-central Appalachian and southwest Amazonia basement, both of which are distinct from Laurentian Pb isotope compositions. The boundary between Amazonian (southern Appalachian) and Laurentian (northern Appalachian) Pb isotope compositions is thus a terrane boundary, with Geon 13 magmatic rocks being the youngest common crustal component. In comparison, the Paraguá block of the southwestern margin of Amazonia consists of a Geon 19–16 basement complex intruded by the batholithic-scale Geon 13 San Ignacio granite suite. The latter also contains inherited Geon 19–16 zircon and has Pb isotope compositions that help define the Amazonian trend. The correspondence of magmatic, inherited, and detrital ages and similarity in Pb isotope compositions are consistent with an origin for the exotic/orphaned Mars Hill terrane as an outboard sliver of the Paraguá block that developed before Grenvillian orogenesis (Geons 12–9). Manifestations of the latter are concentrated around the margins of the Paraguá block in the Sunsás (southwest), Nova Brasilândia (north), and Aguapeí belts (east). The Sunsás belt is a mostly low-grade metasedimentary belt with only minor Geon 10–9 magmatism and no Geon 12 or 11 magmatism, thus distinguishing it from the Mars Hill terrane. The Arequipa-Antofalla terrane, exposed in Andes basement inliers, lies outboard of the Sunsás belt and has Pb isotope and geochronologic characteristics that permit a correlation with the Mars Hill terrane and a paleogeographic position between the Mars Hill terrane and the Sunsás belt. The histories of the Mars Hill terrane, Arequipa-Antofalla terrane, and Paraguá block merge during Geons 10–9 and final collisional orogenesis between southeast Laurentia and southwestern Amazonia.

ABSTRACT The amalgamation of Laurentia’s Archean provinces ca. 1830 Ma was followed by ~700 m.y. of accretionary orogenesis along its active southeastern margin, marked by subduction of oceanic lithosphere, formation of arcs and back-arcs, and episodic accretion. This prolonged period of active-margin tectonic processes, spanning the late Paleoproterozoic and Mesoproterozoic eras, resulted in major accretionary crustal growth and was terminated by closure of the Unimos Ocean (new name). Ocean closure was associated with rapid motion of Laurentia toward the equator and resulted in continental collision that led to profound reworking of much of the accreted Proterozoic crust during the ca. 1090–980 Ma Grenvillian orogeny. The Grenvillian orogeny resulted in formation of a large, hot, long-duration orogen with a substantial orogenic plateau that underwent extensional orogenic collapse before rejuvenation and formation of the Grenville Front tectonic zone. The Grenvillian orogeny also caused the termination and inversion of the Midcontinent Rift, which, had it continued, would likely have split Laurentia into distinct continental blocks. Voluminous mafic magmatic activity in the Midcontinent Rift ca. 1108–1090 Ma was contemporaneous with magmatism in the Southwestern Laurentia large igneous province. We discuss a potential link between prolonged subduction of oceanic lithosphere beneath southeast Laurentia in the Mesoproterozoic and the initiation of this voluminous mafic magmatism. In this hypothesis, subducted water in dense, hydrous Mg-silicates transported to the bottom of the upper mantle led to hydration and increased buoyancy, resulting in upwelling, decompression melting, and intraplate magmatism. Coeval collisional orogenesis in several continents, including Amazonia and Kalahari, ties the Grenvillian orogeny to the amalgamation of multiple Proterozoic continents in the supercontinent Rodinia. These orogenic events collectively constituted a major turning point in both Laurentian and global tectonics. The ensuing paleogeographic configuration, and that which followed during Rodinia’s extended breakup, set the stage for Earth system evolution through the Neoproterozoic Era.

ABSTRACT Neoproterozoic to Cambrian isolation of Laurentia during the breakup of Rodinia was associated with multiple large igneous provinces, protracted multiphase rifting, and variable subsidence histories along different margin segments. In this contribution, we develop a paleogeographic model for the Neoproterozoic tectonic evolution of Laurentia based on available stratigraphic, paleomagnetic, petrologic, geochronologic, and thermochronologic data. Early Tonian strata are confined to intracontinental basins in northern Laurentia. Breakup of Rodinia around Laurentia began in earnest with emplacement of the ca. 778 Ma Gunbarrel large igneous province, interpreted to have accompanied separation of the North China block along the Yukon promontory, and onset of localized, intracratonic extension southward along the western margin. Eruption of the ca. 760–740 Ma Mount Rogers volcanic complex along the Southern Appalachian segment of the eastern margin may record extension associated with separation of the Kalahari or South American terranes. At about the same time, the Australia-Mawson blocks began separating from the Sonoran segment of the southern margin and Mojave promontory. Emplacement of the ca. 720 Ma Franklin large igneous province along the northern margin was likely associated with separation of Siberia and was followed by widespread bimodal volcanism and extension along the western margin spanning ca. 720–670 Ma, leading to partial separation of continental fragments, possibly including Tasmania, Zealandia, and Tarim. Emplacement of the ca. 615 Ma Central Iapetus magmatic province along the eastern margin marked rifting that led to separation of Baltica and Amazonia, and partial separation of the Arequipa-Pampia-Antofalla fragments. During the late Ediacaran to Cambrian, the western, northern, eastern, and southern margins all experienced a second episode of local extension and mafic magmatism, including emplacement of the ca. 585 Ma Grenville dikes and ca. 540–532 Ma Wichita large igneous province, leading to final separation of continental fragments and Cambrian rift-drift transitions on each margin. Cryogenian rifting on the western and northern margins and segments of the eastern margin was contemporaneous with low-latitude glaciation. Sturtian and Marinoan glacial deposits and their distinctive ca. 660 Ma and 635 Ma cap carbonates provide important event horizons that are correlated around the western and northern margins. Evidence for Ediacaran glaciation is absent on Laurentia, with the exception of glacial deposits in Scotland, and putative glacial deposits in Virginia, which both formed on the poleward edge of Laurentia. Patterns of exhumation and deposition on the craton display spatial variability, likely controlled by the impingement of mantle plumes associated with mantle upwelling and extensional basin formation during the piecemeal breakup of Rodinia. Glaciation and eustasy were secondary drivers for the distribution of erosion and Neoproterozoic sedimentation on North America.

ABSTRACT The passive margins of Laurentia that formed during Neoproterozoic–Cambrian breakup of the supercontinent Rodinia record subsequent histories of contraction and translation. This contribution focuses on the northern margin of Laurentia, where recent geologic and geochronologic data have provided new insight into the evolution of northern North America. The Laurentian margin in East and North-East Greenland records synorogenic sedimentation and deformation associated with the Caledonian orogeny—the Silurian to Devonian continent-continent collision between Baltica and Laurentia that followed closure of the northern tract of the Iapetus Ocean. The timing of ultrahigh-pressure metamorphism and simultaneous sinistral and dextral strike-slip faulting in North-East Greenland indicates that the Himalayan-style orogen persisted through the Devonian. In contrast, the Franklinian margin further west records sinistral strike-slip translation of allochthonous crustal blocks and arc fragments starting in the Ordovician–Silurian and culminating with the Devonian–Carboniferous Ellesmerian orogeny, the origin of which remains enigmatic. We suggest that Ellesmerian deformation was related to widespread transpression associated with northward motion of Laurentia during Acadian and Neo-Acadian deformation along the Appalachian margin rather than orthogonal ocean basin closure and microcontinent-continent collision. The Pearya terrane and North Slope subterrane of the Arctic Alaska terrane, separated from the Franklinian passive margin by the Petersen Bay fault and Porcupine shear zone, respectively, best preserve the Paleozoic translational and transpressional history of the northern Laurentian margin. These two major structures record a complex history of terrane accretion and translation that defines the Canadian Arctic transform system, which truncated the Caledonian suture to the east and ultimately propagated early Paleozoic subduction to the Cordilleran margin of western Laurentia.

ABSTRACT The North American continent has a rich geologic record that preserves evidence for tectonic processes throughout much of Earth’s history. Within this long history, however, particular times—e.g., “turning points”—have had specific and lasting impact on the evolution of Laurentia (ancestral North America). This volume is focused on seven of these “turning points”: (1) The Neoarchean (2.7–2.5 Ga), characterized by cratonization and the Kenoran orogen(s); (2) the Paleoproterozoic (1.9–1.7 Ga) and the initial assembly of Laurentia; (3) the Mesoproterozoic (1.5–1.4 Ga) Andean-style margin on the southern edge of Laurentia with the Pinware-Baraboo-Picuris orogeny; (4) the 1.2–1.0 Ga Midcontinent rift, and the Grenville orogeny and assembly of Rodinia; (5) the 700–500 Ma Neoproterozoic breakup of Rodinia; (6) the mid-Paleozoic (420–340 Ma) closure of the Iapetus and Rheic oceans and the development of the Appalachian-Caledonian orogen; and (7) the Jurassic–Paleogene (200–50 Ma) assembly of the North American Cordilleran margin by terrane accretion and subduction. The assembled chapters provide syntheses of current understanding of the geologic evolution of Laurentia and North America, as well as new hypotheses for testing. The inclusion of work from different geological time periods within a single volume provides continent-wide perspectives on the evolution of tectonic events and processes that acted on and within Laurentia.

The North American continent has a rich record of the tectonic environments and processes that occur throughout much of Earth history. This Memoir focuses on seven “turning points” that had specific and lasting impacts on the evolution of Laurentia: (1) The Neoarchean, characterized by cratonization; (2) the Paleoproterozoic and the initial assembly of Laurentia; (3) the Mesoproterozoic southern margin of Laurentia; (4) the Midcontinent rift and the Grenville orogeny; (5) the Neoproterozoic breakup of Rodinia; (6) the mid-Paleozoic phases of the Appalachian-Caledonian orogen; and (7) the Jurassic–Paleogene assembly of the North American Cordillera. The chapters in this Memoir provide syntheses of current understanding of the geologic evolution of Laurentia and North America, as well as new hypotheses for testing.